Process and unit for the separation of air by cryogenic distillation

ABSTRACT

An air separation unit having a medium-pressure column, a low-pressure column, a chamber, a heat exchanger, a bottom condenser of the low-pressure column and a condenser placed in the chamber is provided. The air separation unit also includes an expansion device configured to expand oxygen-rich liquid from the bottom of the low-pressure column before the oxygen-rich liquid is introduced to the chamber and a compressor configured to compress the gas from the chamber, wherein the compressor is downstream of the chamber and upstream of the low-pressure column.

The present invention relates to a process and to a unit for separating air by cryogenic distillation.

It is known to separate air in a unit comprising one medium-pressure column and two low-pressure columns operating at the same pressure, one of the low-pressure columns being fed at the top with the bottoms liquid from the other column and each low-pressure column having a bottom condenser.

One objective of the invention is to reduce the separation energy for producing impure oxygen, in particular in the case where there is no co-production of nitrogen.

Another objective of the invention is to reduce the cost of at least some elements of the unit.

All the percentages relating to purities are molar percentages.

The invention involves the use of a cold compressor for compressing an oxygen-rich gas, originating from a chamber operating at a pressure below that of the low-pressure column, the gas being intended for the bottom of a low-pressure column. This makes it possible to decouple the pressure at the bottom of the medium-pressure column with the top of the low-pressure column.

The invention is particularly advantageous for the case where air partially condenses in the condenser of the chamber operating at lower pressure than the low-pressure column.

According to one subject of the invention, a process is provided for separating air by cryogenic distillation, wherein:

-   -   i) a stream of compressed and purified air is cooled in a heat         exchanger and sent to a column operating at medium pressure;     -   ii) the stream of air is separated into a nitrogen-enriched         stream and an oxygen-enriched stream;     -   iii) a portion of the nitrogen-enriched stream is sent to a         low-pressure column;     -   iv) at least one portion of the oxygen-enriched stream is sent         to the low-pressure column;     -   v) a nitrogen-rich stream is withdrawn from the top of the         low-pressure column;     -   vi) an oxygen-rich stream is withdrawn from the bottom of the         low-pressure column and sent to a chamber containing at least         one condenser-reboiler;     -   vii) a gaseous stream originating from the chamber is withdrawn         therefrom and sent back to the first low-pressure column,         preferably at the bottom;     -   viii) a portion of the nitrogen-enriched stream from step ii)         condenses at least partially in a condenser fed by a bottoms         liquid from the low-pressure column and is sent to the         medium-pressure column and/or the low-pressure column;     -   ix) a stream of warming gas, optionally at least one portion of         the compressed, purified air that is cooled in the heat         exchanger from step i), condenses at least partially in the         condenser-reboiler of the chamber;     -   x) withdrawn from the chamber is a fluid that is richer in         oxygen than the stream withdrawn from the bottom of the         low-pressure column;         characterized in that the oxygen-rich stream withdrawn from the         bottom of the low-pressure column is expanded upstream of the         chamber and the gaseous stream from the chamber is pressurized         upstream of the first low-pressure column.

Preferably:

-   -   the gaseous stream originating from the chamber is compressed in         a compressor having an inlet temperature below −50° C.,         preferably no heating step takes place between the chamber and         the compressor;     -   the oxygen-rich stream withdrawn from the low-pressure column is         expanded to a pressure at most 1 bar below the pressure at the         bottom of the low-pressure column, preferably at most 0.5 bar,         or even at most 0.2 bar below this pressure and/or the gaseous         stream originating from the chamber is compressed in order to         increase its pressure by at most 1 bar, preferably at most 0.5         bar, or even at most 0.2 bar upstream of the low-pressure         column;     -   the chamber does not contain any mass-exchange means, nor does         it contain any packings or distillation plates;     -   the chamber constitutes a second low-pressure column and         contains mass-exchange means, such as packings or distillation         plates, placed at least above the condenser.

According to another subject of the invention, an air separation unit is provided comprising a medium-pressure column, a low-pressure column, a chamber, a heat exchanger, a bottom condenser of the low-pressure column and a condenser placed in the chamber, a line for sending compressed, purified air that is cooled in the heat exchanger to the medium-pressure column, a line for sending a warming gas to the condenser placed in the chamber, a line for sending a nitrogen-enriched gas from the medium-pressure column to the condenser of the low-pressure column, a line for sending an oxygen-enriched stream from the bottom of the medium-pressure column to the low-pressure column, a line for sending oxygen-rich liquid from the bottom of the low-pressure column to the chamber, a line for withdrawing from the chamber a fluid that is richer in oxygen than that sent to the chamber, a line for sending a gas from the chamber back to the low-pressure column, a line for withdrawing an overhead gas from the low-pressure column, characterized in that it comprises an expansion means for expanding the oxygen-rich liquid downstream of the bottom of the low-pressure column and upstream of the chamber and a compressor for compressing the gas from the chamber downstream of the chamber and upstream of the low-pressure column.

Optionally:

-   -   the chamber comprises mass-exchange means above the condenser;     -   the chamber does not comprise any mass-exchange means above the         condenser;     -   the unit comprises a turbine and a line for sending a         nitrogen-rich gas from the medium-pressure column to the         turbine;     -   the unit comprises a pump for pressurizing a stream of liquid         oxygen originating from the low-pressure column and/or from the         chamber upstream of the heat exchanger.

The invention will be described in greater detail with reference to the figures, which represent units according to the invention.

In FIG. 1, the air 1 is compressed between 3 and 5 bar in a compressor 3, purified in a purification unit 5 and split in two. One portion 9 is cooled in the heat exchanger 13 and is sent to the bottom condenser 15 of a chamber 141 where it partially condenses before being sent to the medium-pressure column 39 of a double column.

The double column comprises the medium-pressure column 39 and a low-pressure column 41 which surmounts it, the thermal link between the two columns being provided by a condenser 25 in the bottom of the low-pressure column 41.

The other portion of the air 7 is compressed in a compressor 11, cooled in the heat exchanger 13 and used for vaporizing pressurized liquid oxygen. As the oxygen is vaporized at a low pressure, the vaporization takes place in an external reboiler 27, different from the heat exchanger 13. The liquefied air thus formed is sent to the medium-pressure column 39 after expansion in a valve 19. The liquid air may also be sent to the low-pressure column.

An oxygen-enriched liquid 17 is withdrawn from the bottom of the medium-pressure column 39, cooled in the heat exchanger 43, expanded in a valve and sent to the low-pressure column 41. A liquid 49 having substantially the composition of air is withdrawn at an intermediate level of the medium-pressure column 39, cooled in the heat exchanger 43, expanded in a valve and sent to the low-pressure column 41. A nitrogen-enriched liquid 47 is withdrawn from the top of the medium-pressure column 39, cooled in the heat exchanger 43, expanded in a valve and sent to the top of the low-pressure column 41.

A nitrogen-rich gas 45 is withdrawn from the top of the low-pressure column, heated in the heat exchanger 43 and then in the heat exchanger 13. A portion of this gas may be compressed in the compressor 35 in order to form the stream 37 that participates in the regeneration of the purification unit 5.

A medium-pressure nitrogen stream 33 is withdrawn from the top of the medium-pressure column 39, heated in the heat exchanger 13, expanded in the turbine 23 and heated again in the heat exchanger 13 before being used for the regeneration of the purification unit 5.

An oxygen-rich stream 53 containing between 45 and 75% of oxygen is withdrawn from the bottom of the low-pressure column 41, expanded in a valve 51 and sent to the top of the chamber 141 which, in this variant, is a distillation column with a bottom condenser 15. Found above the condenser are heat- and mass-exchange means 143, for example structured or unstructured packings, or plates. The valve 51 only lowers the pressure of the liquid by 0.15 bar approximately.

The liquid 53 is separated in the chamber in order to form a liquid 29 that is richer in oxygen as bottoms. It is this liquid 29 that is sent to the reboiler 27 after pressurization in the pump 63. A purge liquid 61 is withdrawn from the reboiler 27. Alternatively, an oxygen-rich gas may be withdrawn from the chamber 141.

An overhead gas 145 is withdrawn from the chamber, compressed at the withdrawal temperature in a compressor 21 which increases its pressure by at most 0.15 bar. The gas produced is reinjected into the bottom of the low-pressure column at the outlet pressure of the compressor 21.

With a temperature difference in the heat exchanger 13 of 2° C. at the hot end, a saving of around 2.5% is obtained compared to the same layout without the cold compressor at the bottom of the low-pressure column.

The unit from FIG. 2 differs from that from FIG. 1 in that the chamber 141 does not contain any packings or plates. There is also ascending partial condensation in the reboiler 15. Thus the difference in composition between the liquid 53 sent to the chamber and the liquid 29 withdrawn from the chamber is very reduced even if the liquid 29 is however richer in oxygen than the liquid 53. The gas 145 is the gas produced by partial vaporization of the liquid 53 in the chamber 141 by heat exchange with the air 9.

If the temperature difference at the hot end of the heat exchanger 13 is kept at 2° C., there is a saving of around 1.5% compared to the same layout without an LP bottom cold compressor.

An energy is obtained that is very slightly better than that of the process from WO-A-2007/129152 with the heat exchanger kept at 2° C. at the hot end. Even if a cold compressor is used in the two processes, in the variant of the invention the power of the cold compressor is ten times smaller than in the prior art variant and the nitrogen turbine is two times smaller. It is also observed that the compression ratio in the variant according to the invention is very low and that a technology similar to a fan should suffice for the compressor 21: these elements make it possible to state that the cold compressor 21 and the turbine 23 will be less expensive than in the prior art process.

The cryogenic compression of a fluid that is relatively rich in oxygen should not pose a safety problem.

The concept of compression of the vapor portion in the low-pressure column may be extended to the case of layouts with three condensers in the low-pressure column, with one or two cold compressors to be placed between the three condensers of the low-pressure column. 

1-10. (canceled)
 11. A process for separating air by cryogenic distillation, the process comprising the steps of: i) introducing a cooled stream of compressed and purified air to a column operating at medium pressure; ii) separating the cooled stream of compressed and purified air into a nitrogen-enriched stream and an oxygen-enriched stream; iii) introducing a portion of the nitrogen-enriched stream to a low-pressure column; iv) introducing at least one portion of the oxygen-enriched stream to the low-pressure column; v) withdrawing a nitrogen-rich stream from the top of the low-pressure column; vi) withdrawing an oxygen-rich stream from the bottom of the low-pressure column and sending the oxygen-rich stream to a chamber containing at least one condenser-reboiler; vii) withdrawing a gaseous stream originating from the chamber and sending the gaseous stream originating from the chamber to the low-pressure column; viii) at least partially condensing a portion of the nitrogen-enriched stream from step ii) in a condenser fed by liquid originating from the low-pressure column and sending the at least partially condensed portion to a column selected from the group consisting of the medium-pressure column, the low-pressure column, and combinations thereof; ix) at least partially condensing a stream of warming gas in the condenser-reboiler of the chamber; and x) withdrawing a fluid from the chamber, wherein the fluid is richer in oxygen than the stream withdrawn from the bottom of the low-pressure column, wherein step vi) further comprises expanding the withdrawn oxygen-rich stream to a lower pressure upstream the chamber and downstream the low-pressure column, and wherein step vii) further comprises compressing the gaseous stream originating from the chamber to a higher pressure upstream the low-pressure column and downstream the chamber.
 12. The process as claimed in claim 11, wherein the gaseous stream originating from the chamber is compressed in a compressor having an inlet temperature below −50° C.
 13. The process as claimed in claim 11, wherein the oxygen-rich stream withdrawn from the low-pressure column is expanded to a pressure at most 1 bar below the pressure at the bottom of the low-pressure column, and wherein the gaseous stream originating from the chamber is compressed in order to increase its pressure by at most 1 bar.
 14. The process as claimed in claim 11, wherein the oxygen-rich stream withdrawn from the low-pressure column is expanded to a pressure at most 0.5 bar below the pressure at the bottom of the low-pressure column, and wherein the gaseous stream originating from the chamber is compressed in order to increase its pressure by at most 0.5 bar.
 15. The process as claimed in claim 11, wherein the oxygen-rich stream withdrawn from the low-pressure column is expanded to a pressure at most 0.2 bar below the pressure at the bottom of the low-pressure column, and wherein the gaseous stream originating from the chamber is compressed in order to increase its pressure by at most 0.2 bar.
 16. The process as claimed in claim 11, wherein the chamber comprises an absence of mass-exchange means.
 17. The process as claimed in claim 11, wherein the chamber comprises an absence of packings.
 18. The process as claimed in claim 11, wherein the chamber comprises an absence of distillation plates.
 19. The process as claimed in claim 11, wherein the chamber comprises a second low-pressure column and mass-exchange means.
 20. The process as claimed in claim 19, wherein the mass-exchange means are selected from the group consisting of as packings, distillation plates, and combinations thereof, and wherein the mass-exchange means are disposed above the condenser.
 21. An air separation unit comprising: a medium-pressure column having a top and a bottom; a low-pressure column in fluid communication with the medium-pressure column; a chamber in fluid communication with the low-pressure column and the medium-pressure column; a heat exchanger in fluid communication with the medium-pressure column, the low-pressure column, and the chamber; a bottom condenser disposed in the low-pressure column; a condenser disposed in the chamber; a line configured to send compressed, purified air that is cooled in the heat exchanger to the medium-pressure column; a line configured to transfer a warming gas to the condenser disposed in the chamber; a line configured to transfer a nitrogen-enriched gas from the medium-pressure column to the condenser of the low-pressure column; a line configured to transfer an oxygen-enriched stream from the bottom of the medium-pressure column to the low-pressure column; a line configured to transfer oxygen-rich liquid from the bottom of the low-pressure column to the chamber; a line configured to withdraw a fluid from the chamber that is richer in oxygen than that sent to the chamber; a line configured to transfer a gas from the chamber back to the low-pressure column; a line configured to withdraw an overhead gas from the low-pressure column, wherein the line configured to withdraw the overhead gas from the low-pressure column, comprises an expansion device that is operable to expand the oxygen-rich liquid downstream of the bottom of the low-pressure column and upstream of the chamber; and a compressor operable to compress the gas from the chamber downstream of the chamber and upstream of the low-pressure column.
 22. The unit as claimed in claim 21, wherein the chamber further comprises mass-exchange means, wherein the mass-exchange means are disposed above the condenser.
 23. The process as claimed in claim 22, wherein the mass-exchange means are selected from the group consisting of as packings, distillation plates, and combinations thereof.
 24. The unit as claimed in claim 21, wherein the chamber comprises an absence of mass-exchange means.
 25. The unit as claimed in claim 21 further comprising a turbine and a line configured to transfer a nitrogen-rich gas from the medium-pressure column to the turbine.
 26. The unit as claimed in claim 21 further comprising a pump for pressurizing a stream of liquid oxygen originating from a device disposed upstream of the heat exchanger, wherein the device is selected from the group consisting of the low-pressure column the chamber, and combinations thereof. 